The invention relates to ultraviolet hardening type ink compositions, especially ultraviolet hardening type ink compositions with a temperature-sensitive color change property, in which the color changes reversibly according to a temperature change.
The ultraviolet hardening type ink composition is widely known as a UV ink. Further, various kinds of coloring agents in which the color changes reversibly according to a temperature change are also known, namely, as thermochromic coloring agents.
However, examples using a thermochromic coloring agent as a coloring agent of the UV ink are hardly known, and only a JP, A, 7-324178 is known. This is because the thermochromic coloring agent has the structure that the coloring ingredient is basically included in microcapsules, and due to the fact that it is different in various characteristics compared with the general dye stuffs and pigments, it is considered that there are various restrictions caused by these characteristics.
For example, the following restrictions are cited.
(1) Since the color concentration of a thermochromic coloring agent is extremely low compared with that of the conventional dye stuff and pigment, the large amount is necessarily used in order to get a desired color concentration. However, this hinders the hardening of the UV ink itself and lowers a printing strength after the hardening. Further, this tendency is accelerated by said coloring agent in the form of microcapsules which have an extremely large particle diameter compared with a usual dye stuff and pigment.
(2) Since the light resistance of a thermochromic coloring agent is extremely weak compared with that of the conventional dye stuff and pigment, the addition of an ultraviolet absorber is indispensable. However, since the absorption wave length of such ultraviolet absorber coincides with the one which is necessary for initiating the light hardening of a UV ink, the UV ink including the ultraviolet absorber with a thermochromic coloring agent does not become hardened whatsoever.
In the above-mentioned JP, A, 7-324178, the thermochromic microcapsules containing the ultraviolet absorber having a specific absorption wave length (220-380 nm) and the photopolymerization initiator having a specific absorption wave length (360-450 nm) are used, however, this art requires a sufficient attention for selecting the constituting materials, and can hardly be satisfactory from the practical point of view.
Thus, if a thermochromic coloring agent could be used as a coloring agent for UV ink, various applications thereof could be expected. However, putting it to practical use has not been attained in the state of the art.